Expandable intervertebral implant

ABSTRACT

A joint spacer for therapeutically maintains separation of bones of a joint. A frame defines a longitudinal axis extending between distal and proximal ends. A carriage is slideably retained within the frame and has at least one ramped surface and a threaded portion. An actuator screw is threadably engaged with the threaded portion, and bears against said frame to cause the carriage to slideably move within the frame when the actuator screw is rotated. A first endplate engages a bone of the joint, and has at least one ramped surface that is mateable with the ramped surface of the carriage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. Ser. No.16/144,243, filed on Sep. 27, 2018 (published as U.S. Pat. Pub. No.2019-0021876), which is a continuation application of U.S. patentapplication Ser. No. 13/775,731, filed Feb. 25, 2013 (now U.S. Pat. No.10,117,754), the entire contents of which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

This invention relates to stabilizing adjacent vertebrae of the spine byinserting an intervertebral spacer, and more particularly anintervertebral spacer that is adjustable in height.

BACKGROUND OF THE INVENTION

Bones and bony structures are susceptible to a variety of weaknessesthat can affect their ability to provide support and structure.Weaknesses in bony structures have numerous potential causes, includingdegenerative diseases, tumors, fractures, and dislocations. Advances inmedicine and engineering have provided doctors with a plurality ofdevices and techniques for alleviating or curing these weaknesses.

In some cases, the spinal column requires additional support in order toaddress such weaknesses. One technique for providing support is toinsert a spacer between adjacent vertebrae.

SUMMARY OF THE INVENTION

In accordance with the disclosure, a joint spacer for therapeuticallymaintaining a separation of bones of a joint, comprises: a frame havingdistal and proximal ends defining a longitudinal axis extendingtherebetween; a carriage slideably retained within the frame and havingat least one ramped surface, the carriage further including a threadedportion; an actuator screw threadably engaged with the carriage threadedportion, the actuator screw configured to bear against the frame tocause the carriage to slideably move within the frame when the actuatorscrew is rotated; a first endplate configured to engage a first bone ofthe joint, and having at least one ramped surface mateable with the atleast one carriage ramped surface, whereby when the carriage isslideably moveable by rotation of the actuator screw, the at least oneendplate ramped surface slideable against the at least one carriageramped surface to cause the first endplate to move along an axistransverse to the longitudinal axis to increase a height of the spacer;and a second endplate configured to engage a second bone of the joint.

In an embodiment thereof, the carriage includes at least one additionalramped surface, and the second endplate includes at least one rampedsurface mateable with the at least one additional ramped surface of thecarriage, whereby when the carriage is slideably moved by rotation ofthe actuator screw, the at least one second endplate ramped surfaceslides against the at least one additional carriage ramped surface tocause the second endplate to move along an axis transverse to thelongitudinal axis to increase a height of the spacer.

In other embodiments thereof, the first endplate is configured to abutthe frame as the first endplate is moved along an axis transverse to thelongitudinal axis, whereby the first endplate moves substantially onlyalong an axis transverse to the longitudinal axis; the first endplateincludes at least one aperture through which a fastener may pass tosecure the first endplate to a bone of the joint; the spacer furtherincluding a blocking mechanism to prevent backing out of a fastenerpassed through the first endplate; the first endplate includes one ormore projections configured to engage bone of the joint when the implantis positioned between bones of the joint; at least one of the first andsecond endplates is composed of two interconnected portions ofdissimilar materials; where dissimilar materials are used, one ismetallic and includes at least one aperture through which a fastener maybe passed to attach the implant to a bone of the joint. In anotherembodiment, one dissimilar material is polymeric, and another dissimilarmaterial is metallic.

In further embodiments thereof, the carriage is slideably supported bythe actuator screw and by at least one lateral support extending fromthe carriage to the frame; the spacer further includes a thrust washerinterposed between the actuator screw and the frame; the spacer furtherincludes a polymeric material configured to press against the actuatorscrew to reduce a potential for unintended rotation of the actuatorscrew.

In yet further embodiments, an aperture is formed in part by the firstendplate, and in part by the second endplate, the aperture sized anddimensioned to rotatably support a bone screw when the first endplatehas been moved a distance along the axis transverse to the longitudinalaxis. In another embodiment, the spacer further includes a dovetailconnection formed between the frame and the first endplate when thefirst endplate is configured to abut against the frame.

In another embodiment of the disclosure, a joint spacer fortherapeutically maintaining a separation of bones of a joint, comprisesa frame having distal and proximal ends defining a longitudinal axisextending therebetween; a carriage slideably retained within the frameand having at least one ramped surface, the carriage further including athreaded bore; an actuator screw threadably engaged with the carriagethreaded bore, the actuator screw configured to bear against the frameto cause the carriage to slideably move within the frame when theactuator screw is rotated; a first endplate configured to engage a firstbone of the joint, and having at least one channel having a rampedsurface mateable with the at least one carriage ramped surface, wherebywhen the carriage is slideably moveable by rotation of the actuatorscrew in a first direction, the at least one endplate ramped surfaceslideable against the at least one carriage ramped surface to cause thefirst endplate to move along an axis transverse to the longitudinal axisto increase a height of the spacer; and a second endplate configured toengage a second bone of the joint.

In embodiments thereof, when the actuator screw is rotated in anopposite, second direction, the at least one endplate ramped surface isslideable against the at least one carriage ramped surface to cause thefirst endplate to move along an axis transverse to the longitudinal axisto decrease a height of the spacer; and, the carriage is slideablysupported by the actuator screw and by at least one screw extending fromthe carriage through an elongated channel in the frame.

In yet further embodiments thereof, the first endplate includes ametallic portion having an aperture through which a fastener may bepassed for connecting the implant to body tissue, the first endplatefurther having a polymeric portion connected to the metallic portion,the polymeric portion sized and dimensioned to support a bone of thejoint; and, the frame and the first endplate include mateable dovetailedportions configured to maintain an orientation of the first endplate andthe frame when the first endplate is positioned proximate the frame.

In another embodiment of the disclosure, a method for therapeuticallymaintaining a separation of bones of a joint, comprises: inserting aspacer between bones of the joint, the spacer including—a frame havingdistal and proximal ends defining a longitudinal axis extendingtherebetween; a carriage slideably retained within the frame and havingat least one ramped surface, the carriage further including a threadedbore; an actuator screw threadably engaged with the carriage threadedbore, the actuator screw configured to bear against the frame to causethe carriage to slideably move within the frame when the actuator screwis rotated; a first endplate configured to engage a first bone of thejoint, and having at least one ramped surface mateable with the at leastone carriage ramped surface, whereby when the carriage is slideablymoveable by rotation of the actuator screw, the at least one endplateramped surface slideable against the at least one carriage rampedsurface to cause the first endplate to move along an axis transverse tothe longitudinal axis to increase a height of the spacer; and a secondendplate configured to engage a second bone of the joint; the spacerinserted when the first endplate is positioned proximate the frame; andslideably moving, by rotation of the actuator screw, the at least oneendplate ramped surface against the at least one carriage ramped surfaceto cause the first endplate to move along an axis transverse to thelongitudinal axis to increase a height of the spacer to maintain aseparation of bones of the joint.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a spacer of the disclosure, together with three mountedbone screws;

FIG. 2 depicts the spacer of FIG. 1 , in a compressed or reduced heightconfiguration;

FIG. 3 depicts the spacer of FIG. 1 , in an expanded or increased heightconfiguration;

FIG. 4 depicts a carriage and frame of the spacer of FIG. 1 ;

FIG. 5 depicts an endplate of the spacer of FIG. 1 ;

FIG. 6 depicts a cross-section of the spacer of FIG. 2 ;

FIG. 7 depicts a cross-section of the spacer of FIG. 3 ;

FIG. 8 depicts an exploded view of the spacer of FIG. 1 ;

FIG. 9 depicts a diagrammatic view of aspects of a spacer in accordancewith the disclosure, in a reduced height configuration;

FIG. 10 depicts a the spacer of FIG. 9 , in an expanded heightconfiguration; and

FIG. 11 depicts the spacer of FIG. 1 , implanted between adjacentvertebrae.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments are disclosed herein; however, it isto be understood that the disclosed embodiments are merely examples andthat the systems and methods described below can be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present subject matter in virtually anyappropriately detailed structure and function. Further, the terms andphrases used herein are not intended to be limiting, but rather, toprovide an understandable description of the concepts.

The terms “a” or “an”, as used herein, are defined as one or more thanone. The term plurality, as used herein, is defined as two or more thantwo. The term another, as used herein, is defined as at least a secondor more. The terms “including” and “having,” as used herein, are definedas comprising (i.e., open language).

With reference to FIGS. 1-3 , implant 100 is operative, when positionedbetween adjacent bones of a joint, such as for example vertebrae 10, 12(shown in FIG. 11 ), to stabilize a joint formed by adjacent vertebrae.Implant 100 has a collapsed state or height, illustrated in FIG. 2 , andan expanded state or height, illustrated in FIG. 3 . Implants 100 of thedisclosure may be inset into the intervertebral disc space at acollapsed height, and then expand axially (superior/inferior) to restoreheight loss in the disc space. The implant provides distraction as wellas achieves optimal height restoration. When inserted in a collapsedstated, implants 100 reduce impaction to tissue in the joint spaceduring insertion, and form the least visually blocking or obstructingprofile.

Implant 100 includes two separable endplates 110, 112. A surface 114 ofan endplate 110, 112 can be provided with teeth or other projections 116which can penetrate body tissue to reduce a likelihood of migration ofimplant 100 after implantation. Implant 100 is further secured with oneor more bone screws 300, which pass through bone screw socket 118 withinimplant 100, and into body tissue of the patient. In the embodimentillustrated in FIGS. 1-3 , three sockets 118 for three bone screws areprovided, the bone screws 300 further retained in connection withimplant 100 by blocking fasteners 120. Bone screw 300 can be a polyaxialscrew, and sockets 118 correspondingly shaped, whereby bone screw 300may be inserted into body tissue at an optimal angle with respect toimplant 100, whereby optimal purchase may be obtained, or certain bodytissue may be avoided.

Endplates 110, 112 are moveably connectable to an actuator 150 operableto change a relative relationship of endplates 110 and 112. Actuator 150includes a frame 152 rotatably supporting an actuator screw 154, and amoveable carriage 156. As actuator screw 154 rotates within frame 152,carriage 156 slides within frame 152, driven by cooperation betweenthreads 158 (FIG. 8 ) upon actuator screw 152, and mating threads 160within carriage 156. In the embodiment of FIGS. 1-3 , endplates 110 and112 are formed in two connected portions, including a portion 122, 122Awhich can be polymeric, and a portion 124, 124A, which can be metallic.The portions are joined in the embodiment shown by screws 162, althoughother methods of combining the two connected portions 122, 124 or 122Aand 124A may be used, including a dovetail connection, or adhesive,possibly in combination with each other, or with endplate connectorscrews 162. Metallic portions 124, 124A can provide greater strength forportions of implant 100 which are under relatively greater stress, forexample portions through which a fastener may pass to anchor implant 100within the body. While portions 122, 122A, 124, 124A are described aspolymeric or metallic, it should be understood that other materials maybe used, and that the portions can be of dissimilar materials.

With reference to FIG. 2 , it may be seen that implant 100 is in acompressed state, having a lower height relative to an expanded state,as shown in FIG. 3 . A functioning of device 100 may be best understoodwith reference to FIGS. 9-10 , which correlate with FIGS. 2-3 ,respectively, but which present a simplified view having certainelements eliminated or exaggerated, to ease understanding. Endplates 110and 112 are provided with ramped channels 164, 164A, and an open ramp166, 166A, sized to slidingly receive ramps 168, 168A and 170, 170Adisposed upon carriage 156. While two mating channels and ramps areillustrated for each endplate 110, 120, it should be understood thatone, or more than two, sets of channels and or ramps may be provided.Further, channels 164, 164A may alternatively be formed as ramps.However, a channel can operate to enable a reduction of height, havingan opposing ramp face, whereby rotation of actuator screw 152 in anopposite direction to expansion can drive endplates 110, 112 together,for example when pressure from body tissue is insufficient to collapseendplates 110, 112. Additionally, at least one channel can operate tofoster the maintenance of a connection between carriage 156 and anendplate 110, 112.

Carriage 156 is supported by frame 152 by lateral engagement means, inthis embodiment two support screws 174 engaged with carriage 156, andpassable through respective channels 176 formed in frame 152. Distal end172 of actuator screw 154 provides additional support for carriage 156.Actuator screw 154 is supported by a set screw 178, which passes throughand is rotatably supported within frame 152.

An actuator access port 180 permits passage of a tool, for example a hexdriver (not shown), into engagement with a proximal end 182 of actuatorscrew 152. As actuator screw 152 is turned, distal end 172 bears againsta thrust washer 184, and an end portion of frame 152. As actuator screw152, carriage 156 is driven along actuator screw by interaction ofthreads 158 and 160. As carriage 156 moves, endplates 110, 112 are urgedto move along ramps 168, 168A and 170, 170A, moving relatively apart,and increasing a height of spacer 100. Endplates 110, 112 are preventedfrom moving together with carriage 156 by abutting against an endportion 186 of frame 152. In a given orientation, one of endplate 110and 112 is an upper endplate with respect to an orientation in astanding patient. However, implant 100 may, in some embodiments, beimplantable in either of opposite orientations, and thereforedesignations of upper and lower are provided for ease of understanding,only. It should be understood that only one of endplate 110, 112 may bemoveable with respect to the other. For example, in one embodiment,ramps 168A, 170A may not be provided, and endplate 112 may be attachedto frame 152.

FIG. 11 illustrates a spacer 100 of the disclosure implanted betweenadjacent vertebrae 10, 12. Frame 152 defines a distal or leading end152A which is inserted first into the body, and a proximal or trailingend 152B which passes last into the body, the distal and proximal endsdefining a longitudinal axis extending therebetween. Spacer 100 can beinserted into the body, and into a position between vertebrae, usingminimally invasive methods, for example using a small incision, andspacer 100 may be passed through a cannula or other structure whichmaintains a pathway through body tissue. Spacer 100 may be inserted intothe spinal column through any approach, including anterior,anterolateral, lateral, or posterolateral. A portion of the discannulus, and nucleus pulposus may be removed in order to form a spaceinto which spacer 100 may be inserted. When spacer 100 is in acompressed, or reduced height configuration, dovetail guides 200, 202can be provided to foster maintenance of a relative orientation of upperand lower endplates during insertion or removal of device 100.

Spacer 100 can be inserted configured to have a lower height profile, asshown in FIG. 2 , whereby an extent of distraction of body tissue may bereduced during insertion. Moreover, to the extent that spacer 100 isused to open a pathway towards an implantation site, trauma to adjacenttissue is reduced relative to inserting a spacer having a final heightprofile. Once spacer 100 is positioned between adjacent vertebrae,actuator screw is rotated by a tool. The tool may be positioned entirelywithin the body, or can extend from in interior of the body to outsidethe body, for example having a driving tip at one end and having ahandle at an opposite end, with a shaft extending into the body betweeneach end.

Once actuator screw 154 has been rotated to separate endplates 110, 112a desired amount, the tool is removed. At this point, actuator screw 154may be secured in place, for example using a mechanical block, or anadhesive, to prevent unintended rotation of actuator screw 154. Ascarriage 156 is slideably moved by rotation of actuator screw 154, aramp 166, 166A or a ramped surface of channel 164, 164A of at least oneof endplate 110, 112 slides against at least one ramp 168, 168A, 170, or170A of carriage 156, to cause the endplate to move along an axistransverse to the longitudinal axis of the frame, to increase a heightof the spacer. Rotation of actuator screw 154 in an opposite directioncauses movement along an axis transverse to the longitudinal axis of theframe to decrease a height of the spacer.

Polymeric insets, or a polymeric square nut, for example PEEK, can beprovided, engageable with threads 158 or other portion of actuator screw152, to provide additional friction to prevent height loss under load,particularly under cyclic loading. Similarly, once bone screws 300 havebeen inserted, blocking elements 196 may be rotated to extend over anend of bone screw head 302, preventing screw 300 from backing out. Asimilar mechanical block (not shown) may be provided for actuator screw154.

With reference to FIGS. 1-3, 5-8 , it may be seen that a socket 118 fora polyaxial screw head 302 can be formed entirely within one of upper orlower endplate 110, 112, or may be formed partially within each ofendplate 110 and 112, whereby when spacer 100 has been expanded to afinal height, the proportions of an interior of socket 118 are corrector substantially correct for retaining screw head 302. For example, inFIG. 8 , metallic portion 124 forms an upper portion 190 of socket 118,and mating metallic portion 124A forms a lower portion 192 of socket118. In the embodiment illustrated in the figures, there are threesockets 118, and all are formed of upper and lower portions. However,there may be more or fewer sockets 118, and one or more sockets may beformed entirely in an upper or lower endplate.

In an embodiment, spacer 100 of the disclosure provides an actuator thattranslates relative to the body by means of a threaded actuator screw154. Ramps 168, 168A and 170, 170A on a carrier 152 mate with channels164, 164A, and or ramps 166, on endplates 110, 112. Linear translationof carriage 152 causes endplates 110, 112 to expand implant 100 along anS/I axis with respect to the body. There can be dovetail guides thatcapture endplates 110, 112 when collapsing the implant.

Assembly screws 162 fasten endplates made of dissimilar materials, forexample PEEK polymeric portions 122, 122A to Titanium metallic portions124, 124A. A dovetail and press fit design can be used to connect thedissimilar endplate portions. A PEEK bushing or washer 184 is usedbetween the threaded actuator screw 154 and frame 152 to minimizefriction during expansion of implant 100. Support screws 174 andchannels 176 cooperate to form side or lateral stabilizers, and setscrew 178 supports a nose or leading end of carriage 152. Additionally,cooperating slots and projections (not shown) in carriage 156 and frame152 can be provided for further relative guidance and stability.

In one embodiment, three bone screws 300 are used to provide fixationinto adjacent vertebral bodies, two screws 300 passing through implant100 and into one vertebra, and one screw 300 passing through implant 100into another vertebra, although other combinations may be used. Bonescrews 300 can have spherical or otherwise curved heads, facilitatinginsertion at a desired angle, or may be provided to mate with socket 118in a fixed orientation, particularly depending on a diameter of a neckportion of screw 300. Cam style blocking fasteners 120 can be used toblock bone screws 300 from backing out after being inserted.

Implants of the disclosure enable a continuous expansion and retractionover a range of displacements according to predetermined dimensions of aspecific implant 100 design. This provides the ability to distractvertebral bodies to a desired height, but also to collapse the implant100 for repositioning, if therapeutically advantageous for the patient.Endplates 110, 112 may be shaped to form planes or surfaces whichconverge relative to each, to provide for lordosis, and can be providedwith openings through which bone may grow, and into which bone graftmaterial may be placed. Implant 100 may be used to distract, or forcebones of a joint apart, or may be used to maintain a separation of bonescreated by other means, for example a retractor.

Implant 100 may be fabricated using any biocompatible materials known toone skilled in the art, having sufficient strength, flexibility,resiliency, and durability for the patient, and for the term duringwhich the device is to be implanted. Examples include but are notlimited to metal, such as, for example titanium and chromium alloys;polymers, including for example, PEEK or high molecular weightpolyethylene (HMWPE); and ceramics. There are many other biocompatiblematerials which may be used, including other plastics and metals, aswell as fabrication using living or preserved tissue, includingautograft, allograft, and xenograft material.

Portions or all of the implant may be radiopaque or radiolucent, ormaterials having such properties may be added or incorporated into theimplant to improve imaging of the device during and after implantation.

For example, metallic portions 124, 124A of endplates 110, 112 may bemanufactured from Titanium, or a cobalt-chrome-molybdenum alloy,Co—Cr—Mo, for example as specified in ASTM F1537 (and ISO 5832-12). Thesmooth surfaces may be plasma sprayed with commercially pure titanium,as specified in ASTM F1580, F1978, F1147 and C-633 (and ISO 5832-2).Polymeric portions 122, 122A may be manufactured from ultra-highmolecular weight polyethylene, UHMWPE, for example as specified in ASTMF648 (and ISO 5834-2). In one embodiment, PEEK-OPTIMA (a trademark ofInvibio Ltd Corp, United Kingdom) may be used for one or more componentsof implant 100. For example, polymeric portions 122, 122A can be formedwith PEEK-OPTIMA, which is radiolucent, whereby bony ingrowth may beobserved. Other polymeric materials with suitable flexibility,durability, and biocompatibility may also be used.

In accordance with the invention, implants of various sizes may beprovided to best fit the anatomy of the patient. Components of matchingor divergent sizes may be assembled during the implantation procedure bya medical practitioner as best meets the therapeutic needs of thepatient, the assembly inserted within the body using an insertion tool.Implants of the invention may also be provided with an overall angulargeometry, for example an angular mating disposition of endplates 110,112, to provide for a natural lordosis, or a corrective lordosis, forexample of from 0° to 6° for a cervical application, although muchdifferent values may be advantageous for other joints. Lordotic anglesmay also be formed by shaping one or both of plates 110, 112 to haverelatively non-coplanar surfaces. Expanded implant heights, for use inthe cervical vertebrae for example, may typically range from 7 mm to 12mm, but may be larger or smaller, including as small as 5 mm, and aslarge as 16 mm, although the size is dependent on the patient, and thejoint into which an implant of the invention is to be implanted.Implants 100 may be implanted within any level of the spine, and mayalso be implanted in other joints of the body, including joints of thehand, wrist, elbow, shoulder, hip, knee, ankle, or foot.

In accordance with the invention, a single implant 100 may be used, toprovide stabilization for a weakened joint or joint portion.Alternatively, two, three, or more implants 100 may be used, at a singlejoint level, or in multiple joints. Moreover, implants 100 may becombined with other stabilizing means.

Additionally, implant 100 may be fabricated using material thatbiodegrades in the body during a therapeutically advantageous timeinterval, for example after sufficient bone ingrowth has taken place.Further, implant 100 is advantageously provided with smooth and orrounded exterior surfaces, which reduce a potential for deleteriousmechanical effects on neighboring tissues.

Any surface or component of the invention may be coated with orimpregnated with therapeutic agents, including bone growth, healing,antimicrobial, or drug materials, which may be released at a therapeuticrate, using methods known to those skilled in the art.

Devices of the disclosure provide for adjacent vertebrae to be supportedduring flexion/extension, lateral bending, and axial rotation. In oneembodiment, implant 100 is indicated for spinal arthroplasty in treatingskeletally mature patients with degenerative disc disease, primary orrecurrent disc herniation, spinal stenosis, or spondylosis in thelumbosacral spine (LI-SI). Degenerative disc disease is advantageouslydefined as discogenic back pain with degeneration of the disc confirmedby patient history and radiographic studies, with or without leg(radicular) pain. Patients are advantageously treated, for example, whomay have spondylolisthesis up to Grade 1 at the involved level. Thesurgery position implant 100 may be performed through an Anterior,Anterolateral, Posterolateral, and/or Lateral approach.

In a typical embodiment, implant 100 has a uncompressed height, beforeinsertion, of 12 to 18 mm, and may advantageously be provided incross-sections of 23×32 mm, 26×38 mm and 26×42 mm, with 4, 8, 12, or 16degree lordotic angles, although these are only representative sizes,and substantially smaller or larger sizes can be therapeuticallybeneficial. In one embodiment a spacer 100 in accordance with theinstant disclosure is sized to be inserted using an MIS approach (areduced incision size, with fewer and shorter cuts through body tissue).

Implant 100 may advantageously be used in combination with other knownor hereinafter developed forms of stabilization or fixation, includingfor example rods and plates.

All references cited herein are expressly incorporated by reference intheir entirety. There are many different features to the presentinvention and it is contemplated that these features may be usedtogether or separately. Unless mention was made above to the contrary,it should be noted that all of the accompanying drawings are not toscale. Thus, the invention should not be limited to any particularcombination of features or to a particular application of the invention.Further, it should be understood that variations and modificationswithin the spirit and scope of the invention might occur to thoseskilled in the art to which the invention pertains. Accordingly, allexpedient modifications readily attainable by one versed in the art fromthe disclosure set forth herein that are within the scope and spirit ofthe present invention are to be included as further embodiments of thepresent invention.

What is claimed is:
 1. A joint spacer for therapeutically maintaining aseparation of bones of a joint, comprising: a frame having distal andproximal ends defining a longitudinal axis extending therebetween; acarriage slideably supported in the frame by two support screws engagedwith the carriage and passable through respective channels formed in theframe, the carriage having at least one ramped surface and a threadedportion; an actuator screw threadably engaged with said carriagethreaded portion, said actuator screw configured to bear against saidframe to cause said carriage to slideably move within said frame whensaid actuator screw is rotated; a first endplate configured to engage afirst bone of the joint, and having at least one ramped surface mateablewith said at least one carriage ramped surface; and a second endplateconfigured to engage a second bone of the joint, wherein said carriageincludes at least one additional ramped surface, and said secondendplate includes at least one ramped surface mateable with said atleast one additional ramped surface of said carriage whereby when saidcarriage is slideably moved by rotation of said actuator screw, said atleast one second endplate ramped surface slides against said at leastone additional carriage ramped surface to cause said second endplate tomove along an axis transverse to said longitudinal axis to increase aheight of the spacer, and wherein the first endplate and the secondendplate are capable of moving with respect to the frame.
 2. The jointspacer of claim 1, said first endplate configured to abut said frame assaid first endplate is moved along an axis transverse to saidlongitudinal axis, whereby said first endplate moves substantially onlyalong an axis transverse to said longitudinal axis.
 3. The joint spacerof claim 1, wherein said first endplate includes at least one aperturethrough which a fastener may pass to secure said first endplate to abone of the joint.
 4. The joint spacer of claim 3, further including ablocking mechanism to prevent backing out of a fastener passed throughsaid first endplate.
 5. The joint spacer of claim 1, wherein said firstendplate includes one or more projections configured to engage bone ofthe joint when the implant is positioned between bones of the joint. 6.The joint spacer of claim 1, wherein at least one of said first andsecond endplates is composed of two interconnected portions ofdissimilar materials.
 7. The joint spacer of claim 6, wherein one ofsaid dissimilar materials is metallic and includes at least one aperturethrough which a fastener may be passed to attach the implant to a boneof the joint.
 8. The joint spacer of claim 6, wherein one dissimilarmaterial is polymeric, and another dissimilar material is metallic. 9.The joint spacer of claim 1, wherein said carriage is slideablysupported by said actuator screw and by at least one lateral supportextending from said carriage to said frame.
 10. The joint spacer ofclaim 1, further including a thrust washer interposed between saidactuator screw and said frame.
 11. The joint spacer of claim 1, furtherincluding a polymeric material configured to press against said actuatorscrew to reduce a potential for unintended rotation of said actuatorscrew.
 12. The joint spacer of claim 1, further including an apertureformed in part by said first endplate, and in part by said secondendplate, said aperture sized and dimensioned to rotatably support abone screw when said first endplate has been moved a distance along theaxis transverse to said longitudinal axis.
 13. The joint spacer of claim1, further including a dovetail connection formed between said frame andsaid first endplate when said first endplate is configured to abutagainst said frame.
 14. The joint spacer of claim 1, wherein the spacerhas a length between distal and proximal ends of the spacer along thelongitudinal axis and a width between first and second sides of thespacer, wherein the width is greater than the length of the spacer. 15.A joint spacer for therapeutically maintaining a separation of bones ofa joint, comprising: a frame having distal and proximal ends defining alongitudinal axis extending therebetween; a carriage slideably supportedin the frame by two support screws engaged with the carriage andpassable through respective channels formed in the frame, the carriagehaving at least one ramped surface and a threaded bore; an actuatorscrew threadably engaged with said carriage threaded bore, said actuatorscrew configured to bear against said frame to cause said carriage toslideably move within said frame when said actuator screw is rotated; afirst endplate configured to engage a first bone of the joint, andhaving at least one channel having a ramped surface mateable with saidat least one carriage ramped surface, whereby when said carriage isslideably moveable by rotation of said actuator screw in a firstdirection, said at least one endplate ramped surface slideable againstsaid at least one carriage ramped surface to cause said first endplateto move along an axis transverse to said longitudinal axis to increase aheight of the spacer; and a second endplate configured to engage asecond bone of the joint wherein the first endplate and the secondendplate are capable of moving with respect to the frame.
 16. The spacerof claim 15, wherein when said actuator screw is rotated in an opposite,second direction, said at least one endplate ramped surface is slideableagainst said at least one carriage ramped surface to cause said firstendplate to move along an axis transverse to said longitudinal axis todecrease a height of the spacer.
 17. The spacer of claim 15, whereinsaid carriage is slideably supported by said actuator screw and by atleast one screw extending from said carriage through an elongatedchannel in said frame.
 18. The spacer of claim 15, wherein said firstendplate includes a metallic portion having an aperture through which afastener may be passed for connecting the implant to body tissue, thefirst endplate further having a polymeric portion connected to saidmetallic portion, the polymeric portion sized and dimensioned to supporta bone of the joint.
 19. The spacer of claim 15, wherein said frame andsaid first endplate include mateable dovetailed portions configured tomaintain an orientation of said first endplate and said frame when saidfirst endplate is positioned proximate said frame.